This invention relates to an improved method of producing half-tones in printing and to apparatus used in the method.
In modern printing processes, such as letterpress printing or offset printing, it is not generally possible to directly print shades of gray such as would be seen in a photographic print. Accordingly, in order to produce shades of gray, to reproduce photographs, for example, different sizes of tiny black or dark dots are printed on a white or light background. This technique of printing shades of gray is referred to as printing halftones. In the technique presently used for producing halftones, first a halftone negative is made from the copy, such as a photograph, which is desired to be printed. The halftone negative is made by photographing the copy through a halftone screen, which causes the different shades of gray in the copy to be represented by different sizes of transparent dots on an opaque background.
In offset printing, this negative is then assembled with negatives of the type copy which is desired to print on the same page with the copy containing the shades of gray. The negatives of the type copy, referred to as line negatives, are first made by producing a copy of the type in the form of black letters on a white background such as could be produced, for example, by a typewriter. This copy of black letters on a white background is referred to as cold type. The cold type is cut up and then pasted down together on a carefully laid out sheet in the exact arrangement that the cold type is to be positioned on each page to be printed. In this assembly of cold type, which is referred to as a keyline layout or a mechanical, space is left in the appropriate positions for any halftone copy to be printed on the page. The line negative is then made from the keyline layout. The line negatives are then assembled with the halftone negatives by taping them down with transparent red tape on a sheet of opaque paper, in which the areas for the line negatives to appear and the halftone negatives to appear have been cut out. The assembly of the halftone negatives and the line negatives on the opaque paper is referred to as a flat. The flat is laid over a metal plate, the surface of which has been previously coated with a light sensitive emulsion, and the image from the flat is burned into the emulsion, which hardens under the transparent areas of the flat, rendering them insoluble to water. Nonprinting areas, which are shielded by the opaque parts of the flat, remain soluble. After exposure, the face of the metal plate is then coated with a special ink which adheres to the hardened portions of the emulsion to bring out the printing image but washes away from the nonprinting areas carrying the soluble emulsion with it leaving the metal exposed. The printing image then on the plate is then fixed and the plate is ready for the offset printing press.